Cheap plastic snap caps for 22 rimfire are great but imperfect sometimes. Sometimes they are a bit short or too thin. If demonstrating safely loading a magazine for example, you need a something full length.

These printed ones are a bit better I think for some situations.

Print in a bright coloured material to make it obvious that this is a dummy. Something like bright red is perfect.

You should be able to fit these by hand, use a dollop of glue to keep them in permanently.

I found that variability in depth and thickness between cases from different manufacturers meant that the insert stuck out a little more in one case than another (a RUAG Geco was 0.5mm shorter than a SK Standard Plus). Both perfectly within spec for a .22LR (overall length 25.4mm), but worth noting.